Large-amplitude conformational changes in proteins, including loop motions, relative motions between domains, collective breathing of protein cores, ligand-binding or oligomerization reactions, and overall folding-unfolding events, may be closely coupled, and in some instances rate-limiting, to biological functions such as molecular recognition, transitions along the catalytic cycle of enzymes, and inhibition or activation of proteins through intra-or inter-molecular protein-protein interactions. Mutations that perturb dynamical processes and conformational equilibria are associated with significant pathology, including loss or gain of function and misfolding. Recent developments, including those from the applicant laboratory, have opened new opportunities for investigation of large amplitude conformational dynamic processes on microsecond-millisecond time scales using NMR spin relaxation measurements at equilibrium in solution and with atomic site resolution, without potential complications introduced by non-native modifications necessary for other solution-state spectroscopic techniques. The proposed research has four primary objectives: (1) identification of the mechanistic basis for cell adhesion mediated by domain (strand) swapping in the cadherin superfamily, (2) assessment of the role of conformational mobility in catalysis by the essential enzyme ornithine 5'-monophosphate decarboxylase, (3) elucidation of the folding mechanism and description of the unfolded-state ensemble for the villin headpiece domain HP67, and (4) development of novel experimental and theoretical methods for characterizing protein dynamics on s-ms time scales. Time-dependent structural changes underlie the normal function of proteins, and misfunction in genetic diseases, cancer, and other pathologies; the proposed research will quantify this linkage for three protein systems involved in cellular structure and basic metabolism. Completion of these goals will enable additional future applications to a wide range of protein systems of biological importance.